1. Field of Invention
The invention relates to a wire cut electric discharge machine that performs electric discharge machining on a workpiece placed fixedly in a machining tank and, more particularly, to a wire cut electric discharge machine that performs electric discharge machining using a wire electrode stretched between an upper arm disposed at an upper portion of a machining tank and a lower arm slidably inserted into the machining tank.
2. Description of Related Art
In a wire cut electric discharge machine as described, a seal structure using an O-ring has been conventionally employed to allow the lower arm, inserted into the machining tank, to be sealed against leakage from the machining tank and to be slidable relative to the machining tank.
FIG. 6 is a sectional view showing a conventional seal structure for use in a wire cut electric discharge machine. An XY table 36 moves a workpiece W together with a machining tank 33, on an X-Y plane. Thus, while a lower arm 42, made of stainless steel or other materials, is inserted into the machining tank 33 so as to be movable, in the X and the Y directions, relative to the machining tank 33, the lower arm 42 must be sealed against leakage from the machining tank 33.
In this seal structure 51, a housing 53, as a support member, is attached to the machining tank 33 so as to be movable, relative to the machining tank, in a direction perpendicular to the sheet surface of FIG. 6 (hereinafter referred to as the xe2x80x9cX-directionxe2x80x9d) and sealed against leakage from the machining tank 33. The lower arm 42 is mounted to the housing 53 with intervention by an O-ring 55 and a seal packing 57. In this case, the lower arm 42 is movable, relative to the machining tank 33, in a left-right direction (referred to as the xe2x80x9cY-directionxe2x80x9d hereinafter) in FIG. 6 because of sliding contact between the lower arm 42 and the seal packing 57. At the same time, the lower arm 42 is sealed, with the O-ring 55 and the seal packing 57, from the machining tank 33. The housing 53 has a groove 59, which is rectangular in cross section and formed all along its inner circumference. The O-ring 55 is fitted into the groove 59 so as to be prevented from being released from the housing 53 when the lower arm 42 slides relative to the machining tank 33.
In the conventional housing 53, as shown in FIG. 7A, the O-ring 55 and the seal packing 57 are sandwiched between a bottom surface 59a of the groove 59 and the outer circumference of the lower arm 42 to ensure that the lower arm 42 is sealed against leakage from the machining tank 33. The distance between sidewalls 59b of the groove 59 is intentionally made greater than the thickness of the O-ring 55. Accordingly, when the outside diameter of the lower arm 42 is small, a clearance is provided between the groove 59 and the O-ring 55, as shown in FIG. 7B, and the machining fluid 35 leaks from the clearance.
On the other hand, when the outside diameter of the lower arm 42 is large, the O-ring 55, sandwiched between the bottom surface 59a and the seal packing 57, is greatly compressed, as shown in FIG. 7C. In this case, a great resilient force is generated from the O-ring 55 to the outer circumference of the lower arm 42, resulting in inferior sliding of the lower arm 42.
Accordingly, in the conventional wire cut electric discharge machine, the lower arm 42 must be manufactured, by undergoing surface finish cutting, within extremely fine tolerances so as to be, for example, 85 mm in diameter with a tolerance of xc2x10.1 mm.
In view of the foregoing problems, the invention provides a wire cut electric discharge machine that ensures smooth sliding of a lower arm relative to a machining tank and fluid-tightness between the lower arm and the machining tank, even when the diameter of the lower arm has a rough tolerance.
According to one aspect of the invention, a wire cut electric discharge machine includes a machining tank storing a machining fluid, an upper arm disposed at an upper portion of the machining tank, a lower arm inserted into the machining tank, a wire electrode extending between the upper arm and the lower arm for performing electric discharge machining on a workpiece placed fixedly in the machining tank, a support member mounted to the machining tank for supporting the lower arm such that the lower arm is slidable relative to the machining tank and sealed from the machining tank, and an O-ring interposed between the lower arm and the support member. The support member has a groove formed all along its inner circumference, and the O-ring makes contact with a pair of sidewalls of the groove so that the lower arm is kept sealed from the machining tank.
In this structure, even when the outside diameter of the O-ring varies depending on an error in diameter of the lower arm, contact between the sidewalls and the O-ring does not change greatly and fluid-tightness can be ensured. In addition, because the O-ring does not need to be sandwiched between a bottom surface of the groove and the outer circumference of the lower arm, the groove may be designed to be relatively deep so that smooth sliding of the lower arm can be ensured even when the outside diameter of the lower arm is large.
Accordingly, even when the diameter of the lower arm has a rough tolerance, smooth sliding of the lower arm and fluid-tightness between the lower arm and the machining tank can be reliably ensured. Thus, commercially available pipes can be used, without undergoing finish cutting, as the lower arm. As a result, the manufacturing cost of the wire cut electric discharge machine can be significantly reduced.
According to another aspect of the invention, a clearance is provided between the O-ring and the bottom surface of the groove.
Thus, even when the outside diameter of the lower arm is larger than the design value, smooth sliding of the lower arm can be reliably ensured. Accordingly, smooth sliding of the lower arm can be ensured more reliably in spite of variations in the outside diameter of the lower arm.
According to another aspect of the invention, a seal packing is provided between the O-ring and the outer circumference of the lower arm. Thus, smoother sliding of the lower arm can be ensured.
According to another aspect of the invention, the seal packing is L-shaped, and one side of the seal packing makes contact with the outer circumference of the lower arm and another side thereof makes contact with one of the sidewalls of the groove.
Because one side of the seal packing is sandwiched between the O-ring and the outer circumference of the lower arm, smoother sliding of the lower arm can be ensured. In addition, because another side of the seal packing is in contact with one of the sidewalls, the O-ring is unlikely to be released from the groove. In other words, because another side of the seal packing overlaps with one of the sidewalls of the groove, the O-ring hardly gets released from the overlapping side. Thus, smooth sliding as well as fluid-tightness of the lower arm is reliably ensured.
According to another aspect of the invention, a plurality of O-rings are arranged side by side along an axial direction of the lower arm and fitted in the groove.
In this case, O-rings having a smaller cross-sectional diameter can be used as compared with the case where a single O-ring is fitted into the groove. As a result, the support member can be downsized outside-diametrically. In addition, the lower arm can be supported reliably by a plurality of O-rings, arranged side by side along the axial direction of the lower arm, even when a force exerted perpendicularly to the axial direction of the lower arm, that is, a twisting force is applied to the lower arm. Thus, the wire cut electric discharge machine can be reduced in size, and a twisting force against the lower arm can be effectively cancelled.
According to another aspect of the invention, a seal packing is provided between at least one O-ring and the outer circumference of the lower arm. In addition, the groove is formed into two steps and a step adjacent to the seal packing is wider than a step adjacent to at least one O-ring.
In this case, smoother sliding of the lower arm can be ensured by the seal packing. Additionally, a seal packing having a width equal to or greater than the width of the O-ring or the sum of the widths of a plurality of O-rings can be used. A wide seal packing can more reliably prevent at least one O-ring from getting released from the groove. Consequently, smooth sliding as well as fluid-tightness of the lower arm is reliably ensured.
According to still another aspect of the invention, the groove has a width slightly smaller than the sum of cross-sectional diameters of the at least one O-ring. Thus, the at least one O-ring is compressed and fitted between a pair of sidewalls of the groove to prevent leakage of the machining fluid. Even when the outside diameter of the at least one O-ring varies depending on an error in diameter of the lower arm, contact between the sidewalls and the at least one O-ring does not change greatly and, as a result, fluid-tightness is ensured.
According to still another aspect of the invention, the groove has a depth substantially equal to a cross-sectional diameter of the O-ring. Thus, an error in diameter of the lower arm is within the cross-sectional diameter of the O-ring, and the lower arm can be movably supported relative to the machining tank and sealed from the machining tank.